The present invention relates to a printing process and a printing apparatus for forming a printing image directly on a printing medium, more specifically, the present invention relates to an ink jet printing process and a printing apparatus, where a printing image is directly formed on a printing medium by an ink jet method of ejecting an oil ink using electrostatic field and where high-quality printing image and high-speed printing can be obtained.
The printing process for forming a printing image on a printing medium based on image data signals includes an electrophotographic method, a sublimation-type or melting-type heat-transfer method and an ink jet method.
The electrophotographic method requires a process of forming an electrostatic latent image on a photoreceptor drum by electrification and exposure and therefore, suffers from complicated system and expensive apparatus.
The heat-transfer method uses an ink ribbon and therefore, despite its inexpensive apparatus, suffers from high running cost and treatment of a waste material.
The ink jet method performs the printing directly on a printing medium by ejecting an ink only on a desired image area using an inexpensive apparatus and therefore, ensures efficient use of coloring material and low running cost.
With respect to the method for applying the ink jet technology to printing system, for example, JP-A-10-286939 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) discloses a process for additionally printing variable numbers, marks or the like on the same printing paper using the ink jet system by providing an ink jet printing apparatus to a rotary printing press.
The printing of image information is preferably in a level as high as comparable to the photographic image, however, conventional ink technologies of pressure-ejecting an aqueous or organic solvent-type ink containing a dye or pigment as a coloring material is disadvantageous in that since a droplet containing a large amount of a solvent is ejected, unless expensive exclusive paper is used, the printing image blurs.
Accordingly, in the case of performing the printing on a normal printing paper sheet or a non-absorptive medium such as plastic sheet, a high-quality printing image cannot be obtained.
As one of the ink jet technologies, a method of heat-melting an ink which is solid at an ordinary temperature, and jetting out the obtained liquid ink to form an image is known. When this ink is used, blurring of the printing image may be reduced, however, because of high viscosity of the ink at the ejection, a fine droplet cannot be jetted out and the obtained individual dot images are large in both the area and the thickness, as a result, a high-precision image cannot be formed.
In recent years, an ink jet method of ejecting an oil ink using electrostatic field has been proposed. In this ink jet method by the ejection of an oil ink, ink stirring means is provided in the ink tank so as to prevent the precipitation and coagulation of the oil ink. The stirring means used is a circulation pump, a stirring blade, an undulator or the like. In the case of a circulation pump, a pump for exclusive use of stirring is generally provided. Since a liquid feed pump for feeding an ink to the ejection head is provided, at least 2 pumps are provided and this is one obstacle to the simplification, miniaturization and reduction in the cost.
The present invention has been made by taking account of the above-described problems and the object of the present invention is to provide an ink jet printing process capable of printing a printed matter having a clear and high-quality image by an inexpensive apparatus and a simple and easy method, where the feed of ink to the ejection head and the prevention of precipitation and coagulation of ink can be attained at the same time by a simpler construction than in conventional processes.
In order to attain this object, according to the invention of an ink jet printing process described in claim 1, an ink jet printing process for forming an image directly on a printing medium by an electrostatic ink jet method of ejecting an oil ink using electrostatic field based on signals of image data and preparing a printed matter by fixing said image is provided, wherein the process uses an ink tank for storing the oil ink, an ink circulation line for stirring an oil ink stored in the ink tank, and an ink feed line for feeding the oil ink to an ink jet ejection head, branched from the ink circulation line, and wherein, an ink is circulated to the ink circulation line to simultaneously perform the stirring of ink stored in the ink tank and the feeding of ink to the ink jet ejection head.
According to the invention described in claim 2, in the ink jet printing process of claim 1, an ink recovery line for recovering the oil ink from the ink jet ejection head, connected to the ink circulation line, is provided and an ink is circulated to the ink circulation line to recover the ink from the ink jet ejection head.
According to the invention described in claim 3, in the ink jet printing process of claim 1 or 2, the oil ink is obtained by dispersing at least colored particles in a nonaqueous solvent having an electric resistivity of 109 xcexa9cm or more and a dielectric constant of 3.5 or less.
According to the invention of an ink jet printing apparatus described in claim 4, an ink jet printing apparatus comprising image-forming means of forming an image directly on a printing medium based on signals of image data and image-fixing means of fixing the image formed by the image-forming means to obtain a printed matter, the image-forming means being an ink jet drawing device of ejecting an oil ink from an ejection head using electrostatic field, is provided, wherein the image-forming means comprises an ink jet ejection head, ink feed means of feeding the oil ink to the ink jet ejection head, an ink tank for storing the oil ink and ink circulation means of stirring an oil ink stored in the ink tank, and the ink feed means is branched from the ink circulation means.
According to the invention described in claim 5, in the ink jet printing apparatus of claim 4, ink recovery means of recovering the oil ink from the ink jet ejection head is provided and the ink recovery means is connected to the ink circulation means.
According to the invention described in claim 6, in the ink jet printing apparatus of claim 4 or 5, the oil ink is obtained by dispersing at least colored particles in a nonaqueous solvent having an electric resistivity of 109 xcexa9cm or more and a dielectric constant of 3.5 or less.
According to the invention described in claim 7, in the ink jet printing apparatus in any one of claims 4 to 6, the image-forming means has a fixing apparatus for fixing the ink.
According to the invention described in claim 8, the ink jet printing apparatus in any one of claims 4 to 8 comprises dust-removing means of removing dusts present on the surface of the printing medium before and/or during the printing on the printing medium.
According to the invention described in claim 9, in the ink jet printing apparatus in any one of claims 4 to 8, at the time of drawing an image on the printing medium, the drawing is performed by rotating an opposing drum disposed at the position facing the ejection head through the printing medium and thereby moving the printing medium.
According to the invention described in claim 10, in the ink jet printing apparatus of claim 9, the ejection head comprises a single channel head or a multi-channel head and the drawing is performed by moving the head in the direction parallel to the axis of the opposing drum.
According to the invention described in claim 11, in the ink jet printing apparatus in any one of claims 4 to 7, at the time of drawing an image on the printing medium, the drawing is performed by running the printing medium while interposing and holding it between at least a pair of capstan rollers.
According to the invention described in claim 12, in the ink jet printing apparatus of claim 11, the ejection head comprises a single channel head or a multi-channel head and the drawing is performed by moving the ejection head in the direction orthogonal to the running direction of the printing medium.
According to the invention described in claim 13, in the ink jet printing apparatus of claim 9 or 11, the ejection head comprises a full line head having almost the same length as the width of the printing medium.
According to the invention described in claim 14, in the ink jet printing apparatus in any one of claims 4 to 13, the ink jet drawing device has ink temperature-controlling means of controlling the temperature of the oil ink in the ink tank for storing the oil ink.
According to the invention described in claim 15, in the ink jet printing apparatus in any one of claims 4 to 14, the ink jet drawing device has concentration-controlling means of controlling the concentration of the oil ink.
According to the invention described in claim 16, the ink jet printing apparatus in any one of claims 4 to 15 comprises cleaning means of cleaning the ejection head.